The molecular basis of cystic fibrosis, the most common lethal genetic disease of the Caucasians, is totally obscure. Two aspects of the disease are being studied: (1) the peculiar association of the mucoid variants of pseudomonas aeruginosa with CF patients, a bacteriological association almost pathognomonic of CF, and (2) the existence of low molecular weight membrane-active polypeptides with ionophore-like activity identified in fluids such as urine, serum, and the medium of CF fibroblasts in tissue culture.